19. The relationship between plasmids and bacteria is said to be endosymbiotic. This is because
a. the plasmids exist on the surface of the bacterium
b. the bacterium exists within the plasmid
c. the bacterium is helped by the relationship, but the plasmid suffers
d. the plasmid exists within the bacterium and the relationship of bacterium and plasmid is mutually beneficial
e. the plasmid produces nutrients, which are used by the bacterium